One of the best sites for categorical photographs is
http://www.iti.cs.tu-bs.de/TI-INFO/koslowj/PHOTOS/math.html

I just noticed some photographs taken at  CT2000 - not many of mathematicians, but there are photographs of the amazing room in which the lectures were given:
http://www.maths.mq.edu.au/~steffen/Photos/00-07-15/


Another site with photographs of mathematicians:
http://andrej.com/mathematicians/